1. Field of the Invention
The present invention relates to a sheet feeder for moving sheets from a stack and more particularly relates to a simple vacuum assisted impact feeder for removing sheets from the top of such a stack.
2. Prior Art
In the copier art it is often necessary to automatically feed a series of documents or copy sheets from a stack of such sheets along a path of sheet travel to a processing station. In a typical operation, it is necessary that only one sheet be fed at a time from the stack along the paper path. It is therefore a requirement that some mechanism be provided to separate one sheet from the stack and initiate movement of that sheet away from the stack toward the processing station. Once an initial separation has been achieved, other drive mechanisms known in the art can be utilized to rapidly reposition the document or copy sheet for processing. These other sheet handling mechanisms can also be utilized to maintain proper sheet coordination and/or registration with other copier functions. Accordingly, it is often not necessary that the sheet separating mechanism accurately maintain sheet position as it is being separated from the stack.
The prior art sheet separating and movement initiating mechanisms can be roughly categorized as either impact, vacuum assisted, or a combination of impact and vacuum assisted mechanisms. All three sheet separating techniques have been tried with varying degrees of success. Each has its advantages and disadvantages and it is not believed any one of these generic sheet separating mechanisms can be categorically stated to be better than the others.
Those prior art mechanisms employing vacuum assisted separators only, are characterized by a source of vacuum which attracts one sheet away from a stack of such sheets and initiates movement away from the stack. Two examples of such a vacuum assisted sheet transport mechanism are disclosed in U.S. Pat. Nos. 4,121,819 and 4,127,263 to DiFrancesco et al and Wenthe, respectively. Both document transports illustrated in those patents are vacuum assisted transports which feed documents in sequence from the bottom of a stack of those documents. A bottom most sheet is attracted to a vacuum assisted drive roller which then drives the bottom most sheet away from the stack to a separate location for processing.
An impact type transport or feeder is one that relies solely upon frictional forces to engage sheets of paper to be transported and drive those sheets away from the stack. An example of such an impact type feeder is disclosed in U.S. Pat. No. 4,043,549 to Rinehart which has been assigned to the assignee of the present invention. The apparatus disclosed in that patent includes a paddle element which is rotated into contact with a bottom most sheet to initiate movement of that sheet away from the stack. Sheet separation is achieved by angled air jets which reduce the frictional forces between a bottom most and other sheets in the stack. Other examples of impact type only sheet transport mechanisms comprise paddle wheel elements which also intermittently engage a sheet or document to urge that sheet in a particular direction.
An example of a combined impact and vacuum assisted drive mechanism is shown in U.S. Pat. No. 3,998,449 to Hornung. The apparatus disclosed in that patent utilizes an impact device to first separate a bottom most sheet from a stack and then employs a vacuum assist to move a separated sheet away from stack for subsequent processing. Both vacuum and impact device are located on a single rotating drum element which coordinates sheet separation and movement.
Each of the techniques embodied by the aforementioned patents has achieved some degree of success in performing its primary purpose, i.e. sheet separation and movement initialization. Impact only type separators, however, often experience multiple feeds which in turn can lead to sheet jamming at subsequent processing stations. In vacuum assisted mechanisms some techniques must be employed to not only attract single sheets to the vacuum source but also to initiate movement of that sheet once the attraction has caused a sheet separation. To detect jams or to provide movement to a separated but as yet stationary sheet has necessarily made more complex prior sheet separation. It is accordingly at one object of the present invention to provide a simple yet reliable sheet separation and movement initialization mechanism.